Rare-earth-doped fiber and an optical fiber laser using the same

Abstract

A rare-earth-doped fiber includes a center core, made of silica glass into which a rare-earth element is doped; a plurality of outer cores; an inner cladding, disposed around the center core and the outer cores; and an outer cladding, disposed around the inner cladding. The refractive index of the inner cladding is less than that of the inner and outer cores, and the refractive index of the outer cladding is less than that of the inner cladding. At least one of the center core and the outer cores is disposed in an area such that the distance between it and the center of the rare-earth-doped fiber should be greater than 0.71r, where r indicates an outer radius of the inner cladding.

What is claimed is:  
     
         1 . A rare-earth-doped fiber, comprising: 
 a plurality of cores, each made of silica glass doped with a rare earth element;    an inner cladding surrounding all of the plurality of cores and having a refractive index smaller than the refractive index of the plurality of cores;    an outer cladding disposed around the inner cladding and having a refractive index smaller than the refractive index of the inner cladding.    
     
     
         2 . The rare-earth-doped fiber according to  claim 1 , wherein the plurality of cores, each doped with a rare-earth element, are each also doped with germanium or aluminum.  
     
     
         3 . The rare-earth-doped fiber according to  claim 1 , wherein a distance between at least one of the plurality of cores and a central axis of the rare-earth-doped fiber is at least 0.71r , where r is an outer radius of the inner cladding.  
     
     
         4 . The rare-earth-doped fiber according to  claim 3 , wherein the rare earth element is selected from a group consisting of erbium, yttrium, ytterbium, neodymium, holmium, and praseodymium.  
     
     
         5 . The rare-earth-doped fiber according to  claim 4 , wherein each of the plurality of cores is doped with the rare earth element to approximately 1,000 to 40,000 ppm.  
     
     
         6 . The rare-earth-doped fiber according to  claim 3 , wherein the inner cladding is comprised of fluorine-doped silica glass.  
     
     
         7 . The rare-earth-doped fiber according to  claim 3 , wherein the outer cladding is comprised of a fluorine resin.  
     
     
         8 . The rare-earth-doped fiber according to  claim 3 , wherein the diameter of each of the plurality of cores is approximately between 2 and 12 μm.  
     
     
         9 . The rare-earth-doped fiber according to  claim 3 , wherein the outer diameter of the inner cladding is approximately between 100 and 600 μm.  
     
     
         10 . The rare-earth-doped fiber according to  claim 3 , wherein the outer diameter of the outer cladding is approximately between 200 and 800 μm.  
     
     
         11 . The rare-earth-doped fiber according to  claim 3 , wherein the plurality of cores includes a central core disposed along the longitudinal axis of the rare-earth-doped fiber.  
     
     
         12 . The rare-earth-doped fiber according to  claim 11 , wherein the plurality of cores comprises: 
 the central core, and    six outer cores,    wherein the distance between each of the six outer cores and the central axis of the rare-earth-doped fiber is at least 0.71r.    
     
     
         13 . The rare-earth-doped fiber according to  claim 12 , wherein the six outer cores are equally spaced from one another in a circular pattern around the central axis of the rare-earth-doped fiber.  
     
     
         14 . An optical fiber laser, comprising the rare-earth-doped fiber according to any one of claims  1 - 13 .  
     
     
         15 . A method of forming a rare-earth-doped fiber, comprising: 
 providing an inner cladding material;    forming a plurality of through-holes in the inner cladding material parallel to the central axis of the inner cladding material;    inserting a plurality of cores, each made of silica glass doped with a rare earth element, into the through holes in the inner cladding material, the cores having an index of refraction greater than the index of refraction of the inner cladding material;    drawing the inner cladding material, having the plurality of cores inserted therein, until the outer diameter of the inner cladding material reaches a predetermined diameter;    coating the drawn inner cladding material, having the plurality of cores therein, with a resin having a refractive index lower than the refractive index of the inner cladding material.